The invention relates to a method for producing a mirror having at least two mirror surfaces. Furthermore, the invention relates to a mirror of a projection exposure apparatus for microlithography and to a projection exposure apparatus comprising such a mirror.
A projection exposure apparatus for microlithography is usually subdivided into an illumination system and a projection objective. The illumination system produces a desired light distribution for illuminating a pattern of a mask or a reticle. The illuminated pattern is then imaged with extremely high resolution by the projection objective onto a light-sensitive material, and the light-sensitive material is thereby exposed in a manner structured with the pattern. On the basis of the pattern exposed into the light-sensitive material, real structures can be produced in a semiconductor material with the aid of subsequent work steps.
Both the illumination system and the projection objective generally have a multiplicity of optical elements such as, for example, lenses and/or mirrors. In the case of projection exposure apparatus designed for operation at very short wavelengths, for example at a wavelength of less than 100 nm, it becomes necessary to use only mirrors, because no materials are available which have a sufficiently high and at the same time sufficiently homogeneous transmission at those wavelengths, and, therefore, no lenses can be produced for these wavelengths with sufficient quality. The wavelength range below approximately 100 nm is also referred to as the extreme ultraviolet, abbreviated to EUV. Lithography systems that operate in the EUV range are often designed for an operating wavelength of 13.6 nm. Depending on the availability of light sources and optical elements, however, other operating wavelengths can also be used.
In order to enable the pattern to be exposed into the light-sensitive material with high precision, it is necessary for the mirrors used in the projection exposure apparatus to be produced and oriented relative to one another with high precision. Moreover, care must be taken to ensure that, during the operation of the projection exposure apparatus, there is a departure from the precise shaping and the precise orientation of the mirrors only within a permissible tolerance. For these reasons, the mirrors in many cases have a very solid substrate body, which imparts a high mechanical stability to the mirrors. However, as a result of this solid embodiment and the associated large external dimensions of the mirrors, problems with regard to structural space can occur, particularly if the underlying optical design of the projection exposure apparatus requires an arrangement of two mirrors at a small distance back to back with respect to one another.
In this context, DE 10 2005 042 005 A1 discloses, in the case of a high-aperture projection objective for microlithography having an obscured pupil, embodying one mirror as a double mirror in which a respective mirror surface is arranged on a front side and a rear side of a substrate.
However, the misorientations between the two mirror surfaces that arise when a double mirror is manufactured using conventional manufacturing and measuring methods, on account of the accompanying imaging aberrations, require an extensive correction using further mirrors or other optical elements of the projection objective. This can lead to problems particularly when the number of optical elements present is comparatively small and the correction possibilities are thus very limited.
A further problem is that, in known methods for measuring the misorientation between two optical surfaces, for example the two optical surfaces of a lens, the measurement is generally effected through the substrate body. Therefore, for a precise measurement result, stringent requirements have to be made of the substrate body. In the case of a lens, this generally does not cause an additional outlay, since the light used for the exposure also passes through the substrate body and said lens already has to have a high optical quality for this reason. By contrast, in the case of a mirror, the imaging properties of which are characterized by its surface and not by the volume properties of its substrate body, a substrate body which is suitable for a precise measurement in transmission causes a considerable additional outlay and, moreover, results in a significant restriction to the suitable materials.